1. Field of the Invention
This invention relates to a non-aqueous secondary battery, and more particularly to a non-aqueous secondary battery with an improved negative electrode, which has a high energy density, a long charging and discharging cycle life, and good storage stability.
2. Discussion of Background
A lithium secondary battery is expected to be utilized not only as an electric source for portable electronic machines, but also as a high-powered battery for use in electric cars and as an electric power storage device because it has the highest theoretical electromotive force and a high theoretical energy density. However, it appears that no prototype lithium secondary batteries so far announced make the best use of the properties inherent in lithium secondary battery. Moreover, the life span, performance, and energy density of such prototype lithium secondary batteries are not sufficient for practical use. It is considered that one of the most important reasons for these shortcomings is that the performance of the negative electrode is poor.
There are two problems in the practical use of the negative electrode for the lithium secondary battery:
(1) The properties of a negative electrode made of lithium metal are changed while in contact with a solvent because of the high reactivity of the lithium metal. As a result, a repeated use of the lithium secondary battery reduces a battery capacity and shortens a cycle life thereof; and PA1 (2) Lithium metal, which is deposited on the negative electrode of lithium metal by the reduction of a lithium ion in the course of charging, easily turns into needle dendrite, destroying an insulating layer (separator) between the positive electrode and the negative electrode, thus easily causing a short circuit. This leads to the shortening of the cycle life and to the instability of the lithium secondary battery.
In order to solve the above-mentioned problems, there is proposed the use of intercalation compounds, as an active material for the negative electrode, which can incorporate therein metallic lithium or the lithium ion produced while in use. Of such intercalation compounds available at present, Li-GIC (Graphite Intercalation Compound) containing graphite as a host material shows the lowest negative-electrode-reaction electric potential. It is attempted to utilize as such host materials various kinds of carbon materials such as a pyrolytic graphite, and coke in any form, such as a fibrous form or a pulverized form.
It has been suggested that the carbon material generally has a polycrystalline structure, and that (i) crystallite size and shape, (ii) the condition of a crystallite surface, and (iii) the conformation of the crystallites, vary depending on the kinds of the carbon materials. It is difficult to discuss the interrelation between the above-mentioned three conditions for the carbon material and the electrode performance without taking account of the differences in the kinds of the carbon materials. Moreover, no specific principles for the development of the carbon material used in the electrode have been made clear. A negative electrode for use in the lithium secondary battery, which satisfies all conditions for the cycle life, reaction potential, and discharge capacity, has not been developed yet.
There is also proposed that a natural material such as a pitch coke is employed as the negative electrode for use in the lithium secondary battery. However, a lithium secondary battery using such a negative electrode is unstable in performance when repeatedly used, and has not yet been used in practice.